This application is concerned with a study of the nature of kinetics of early events induced by ionizing radiation in bacterial and human tumor cells and suspected to be related closely to the mechanism of cell killing by radiation. It is planned to obtain experimental information important to understanding the still unknown mechanism through which molecular oxygen acts to radiosensitize cells by attempting to characterize some of the fast time processes involved. Upper limits will be established on the lifetime of oxygen-dependent damage at unknown sites in these irradiated cells and on the lifetime of reactive species radiation-induced in the DNA of bacteriophage. Related studies include investigation into the mechanism of radiochemical depletion of intracellular oxygen, the time-diffusion process of oxygen in irradiated cells, and the repair of sublethal damage in HeLa cells irradiated with high intensity pulsed radiation. These experiments will be performed using electron sources which utilize the field emission process to produce electron pulses of high intensity (hundreds of kilorads) and of duration (3 nanoseconds) short compared to the time- scale of at least some of the important post-irradiation intracellular processes. Paralleling the experimental work theoretical models are to be developed dealing with the kinetics of the depletion of intracellular oxygen and of the disappearance of oxygen-sensitive species induced by radiation within the cell. These studies are expected to yield information bearing on the fundamental action of radiation on cells and on the mechanism of the oxygen effect important to the radiotherapy and the radiobiology of cancer.